Keikan jälkeen innostuin ja taputin väkijoukon mukana.

Because jälkeen is a postposition that requires the noun before it to be in the genitive.

• keikka = a gig, show
• keikan = of the gig / gig's form, but in practice just the form used before jälkeen
• jälkeen = after

So:

• keikan jälkeen = after the gig

This is very common in Finnish:

• koulun jälkeen = after school
• ruoan jälkeen = after the food / after the meal
• elokuvan jälkeen = after the movie

Because jälkeen is a postposition, not a preposition.

In English, prepositions come before the noun:

• after the gig

In Finnish, many similar words come after the noun:

• keikan jälkeen

So the structure is literally closer to:

• the gig after

Other common Finnish postpositions work the same way.

Keikka usually means a gig, show, or performance, often in a somewhat informal way.

Depending on context, it could be:

• a concert
• a live performance
• a comedy show
• some other event performance

Compared with konsertti, keikka often sounds a bit more casual and everyday.

So:

• konsertin jälkeen = after the concert
• keikan jälkeen = after the gig / after the show

Innostuin is the past tense, first person singular of innostua.

• innostua = to get excited, to become enthusiastic, to get carried away
• innostuin = I got excited / I became enthusiastic

So in this sentence it suggests a change of state:

• first the speaker was not especially excited
• then, after the gig, they got excited

It is not exactly the same as simply I was excited. It has more of a became excited feeling.

It comes from the verb innostua.

Basic parts:

• infinitive: innostua
• stem used here: innostu-
• past tense marker and personal ending combine into -in

So:

• minä innostuin = I got excited

Some useful comparisons:

• innostun = I get excited / I become enthusiastic
• innostuin = I got excited
• hän innostui = he/she got excited
• me innostuimme = we got excited

Because Finnish often leaves out the subject pronoun when the verb ending already shows who the subject is.

Here both verbs clearly show first person singular:

• innostuin = I got excited
• taputin = I clapped

So minä is unnecessary unless you want emphasis.

You could say:

• Minä innostuin ja taputin...

But it would sound more emphatic, like I got excited.

Taputin is the past tense of taputtaa.

Here it means:

• I clapped

In Finnish, taputtaa can be used without an object when you simply mean clapping/applauding.

So:

• taputin = I clapped / I applauded

If there were an object, it could mean things like:

• taputin koiraa = I patted the dog
• taputin esiintyjää = I applauded the performer

But here no object is needed, because the idea is just that the speaker joined in clapping.

It means along with the crowd or together with the crowd.

Breakdown:

• väkijoukko = crowd
• väkijoukon = genitive form
• mukana = with, along with, along, accompanying

So:

• väkijoukon mukana = with the crowd / along with the crowd

The idea is that the speaker clapped as part of what the crowd was doing.

Because mukana commonly takes a noun in the genitive.

So the pattern is:

• jonkun mukana = with someone / along with someone

Examples:

• ystävän mukana = with a friend
• perheen mukana = with the family
• väkijoukon mukana = along with the crowd

So just like jälkeen, mukana is a word that often comes with a specific case pattern.

Sometimes, but not always in exactly the same way as English with.

In this sentence, mukana means something like:

• along with
• together with
• joining in with

So taputin väkijoukon mukana suggests:

• I clapped along with the crowd
• I joined the crowd in clapping

It is not just a neutral with. It often has a sense of going along with, participating with, or being part of the same action.

Yes, it could be different, but this order is very natural.

Finnish word order is flexible, and the first part of the sentence often sets the scene. Here:

• Keikan jälkeen = after the gig

This gives the time context first, which is very common.

Then comes the main action:

• innostuin ja taputin väkijoukon mukana

You could also say:

• Innostuin keikan jälkeen ja taputin väkijoukon mukana.

But the original version sounds smoother and more natural in ordinary narration.

Grammatically, keikan jälkeen means after the gig.

So the sentence says that after the gig, the speaker got excited and clapped along with the crowd.

In real life, that probably means the clapping happened at the end, once the performance was over, or immediately afterward. Finnish does not force a more precise distinction here. It simply places the whole reaction in the time frame after the gig.

Because the sentence is telling about completed events in the past.

• innostuin = I got excited
• taputin = I clapped

The speaker is narrating what happened. Finnish uses the simple past here, just as English often does:

• After the gig, I got excited and clapped along with the crowd.

There is no need for a separate past progressive form like English was clapping unless the context specifically requires it.